Download Gap Junctions for Engineers: A review

Internal Report 97-01
Gap Junctions for Engineers: A review
Alejandro F Frangi
Division of Intrumentation and Bioengineering
Department of Electronic Engineering
Polytechnic University of Catalonian
March 1997
INDEX
Gap junctions for Engineers: A review
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I. BASIC CONCEPTS
A. C ELLULAR ELEMENTS
B. THE INTRACELLULAR AND EXTRACELLULAR SPACES
C. C ELL J UNCTIONS IN TISSUES
II. GAP JUNCTIONS AND INTERCELLULAR COMMUNICATION
A. B RIEF HISTORICAL BACKGROUND
B. THE STRUCTURE OF G AP JUNCTIONS
C. C ONNEXINS
D. P HYSIOLOGY OF G AP JUNCTIONS
E. G ATING OF G AP JUNCTIONS
F. A GENTS THAT REGULATE P ERMEATION OF G AP JUNCTIONS
G. C ONDUCTANCE OF G AP JUNCTIONS
III. GAP JUNCTIONS AND FAILING HEART
A. C ARDIAC M USCLE TISSUE
B. ISCHEMIA AND M YOCARDIAL I NFARCT
C. G AP JUNCTION ALTERATIONS IN FAILING H EART
IV. ACKNOWLEDGMENT
V. REFERENCES
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I. BASIC CONCEPTS
A. Cellular Elements
Cells, embedded in the extracellular matrix, are the basic units of living tissues.
Although each particular tissue has its characteristic cells it is possible to identify a basic
set of elements that are common to every cell.
A generalized animal cell can be divided in the following main parts:
Plasma (cell) membrane: it is the outer limiting membrane that separates the intracellular
space of the extracellular material and external environment.
Cytosol: by cytoplasm we designate all the material enclosed between the plasma
membrane and the nucleus. The cytosol is the fluid portion of the cytoplasm, i.e., the
intracellular fluid. The cytosol contains ions, small molecules, soluble enzymes and
proteins and nutrients. Organelles and inclusions are also suspended in the cytosol.
Organelles: highly organized structures with characteristic shapes that are highly
specialized for specific cellular activities.
Inclusions: temporary structure that contain secretions and storage products of the cell.
One important cellular element from the structural point of view is the
cytoskeleton. It is a complex network of protein filaments allowing the cell to maintain
its shape and to perform a variety of coordinated cellular movements. There are three
types of filaments that are specialized in particular structures of the cytoskeleton:
microfilaments, microtubules and intermediate filaments.
B. The Intracellular and Extracellular Spaces
Water is one of the main components of body. This water is located in two
compartments: intracellular and extracellular spaces. The intracellular space comprises
approximately 55% of the total water.
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The intracellular liquid (ICL) and extracellular liquid (ECL) have different
composition as shown in the table below reproduced from [Matthews, 1994]:
K+
ICL (mM)
ECL (mM)
125
5
+
12
120
Cl
-
5
125
A-1.223 (1)
108
0
H 2O
55000
55000
Na
Table 1: ICL and ECL composition after [Matthews, 1994]
The extracellular space contains the extracellular matrix (EM). The EM is made up
of two differentiated parts:
• Fundamental substance (or amorphous substance): the main constituents are
proteins and polysaccharides secreted locally by the cells. Dissolved ions are also
present. Glycosaminoglycans (GAG) are a family of non branched polysaccharides
that give the EM its characteristic jelly consistence. The specific GAG involved in
the extracellular space of a given tissue depends on the tissue itself.
• Protein Fibers: which can be of two types depending whether they are filamentous
(collagen, elastine filaments) or adhesive (fibronectine, laminine).
The jelly consistence of the amorphous substance allows easy diffusion of
nutrients, metabolites and hormones arriving from blood to the cells present in tissues.
The filaments give resistance and elasticity to the tissue while adherent proteins have a
cohesive function.
C. Cell Junctions in Tissues
Most of the cells in multicellular organisms are grouped together in cooperative
assemblies called tissues. The mechanisms that allow this assemblies to work properly as
a functional unit or regulate the interaction between the ICL and ECL are called junctions.
On the other hand EM surrounds each cell giving structural cohesion to the tissue.
We can differentiate two types of junctions: cell-cell junctions (CC) and cell-matrix
junctions (CM) depending on the two elements being involved. Having this ideas in
mind, it is possible to establish the following functional classification of cell junctions
[Alberts et al, 1993]:
1
Organic anions.
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1. Occluding Junctions (tight junctions)
They are typical of intestinal epithelium and clearly (hermetically) divide the tissue in two functional
domains (the apical and basolateral surfaces).
2. Anchoring Junctions
2.a. actin filament attachment sites
• adhesion belts (CC)
• focal contacts (CM)
2.b. intermediate filament attachment sites
• desmosomes (CC)
• hemidesmosomes (CM)
3. Communicating Junctions
• gap junctions
• chemical synapses (no physical contact)
• plasmodesmata (plants only)
From the above classification one is acquainted with the diversity of possible cell
junctions. On the other hand, it is clear that only gap junctions provide a mean for
intercellular communication
(physical contact) in animal cells. This special
communicating mechanism is the one that we are to review in this report.
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II. GAP JUNCTIONS AND INTERCELLULAR COMMUNICATION
It is worthy to devote our attention to gap junctions because it is one of the most
widespread communicating mechanism and can be found in large number in almost all
animal tissues and practically all animal species [Alberts et al, 1993]
A. Brief Historical Background
The first demonstration of this type of cell-cell communication comes from the
field of physiology and was carried out in 1958. However, it took almost 10 years before
this physiological coupling was correlated to the presence of gap junctions seen in
electron microscope.
The first studies were performed in nerve cells of crayfish. It was noted that by
applying an electrode in each one of a couple of interacting cells and applying a voltage
step, an unexpectedly large current flowed between them. It was a clear demonstration the
inorganic ions present in the ICL could freely go from one cytoplasmic region to the
other.
Later results showed that small fluorescent dye molecules2 injected into one cell
could likewise pass to the other cell without leaking into the extracellular space provided
that their molecular weight were less that 1kD3.
B. The Structure of Gap Junctions
Gap junctions are formed by a couple of hexameric arrangements of integral
proteins4 that are known as connexons. These arrangements are located in each one of the
interacting cells and join each other in the extracellular space. This tubular arrangement
leaves an aqueous channel in its interior that connects the cytoplasmic regions of adjacent
cells maintaining a relative small separating gap that justifies the name of this junction.
The width of the gap is 2-4 nm.
Connexons are not disperse within the cell. They are usually grouped in clusters
(maculae) with a large number of connexons which are rich in cholesterol [Goodenough
et al, 1996]. In Fig. 1 we show typical dimensions within a gap junction cluster.
Each connexon is made up of six proteins of a multigene family and they receive
the global name of connexins. Connexins are responsible for the different properties of
gap junctions present in different tissues. However, it is possible to identify a certain
common behavior of connexins. In order to understand this point it will be useful to have
a look to the constitution of a general connexin.
2
The most popular is Lucifer Yellow, molecular weight 443 D [Goodenough et al, 1996].
3
Dalton (D). Unit of molecular weight approximately equivalent to the mass of a hydrogen
4
Integral protein: a protein is said to be integral when it has a transmembrane domain.
atom.
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Fig. 1: Two schematic models of gap junctions. A. After [Alberts et al,1993]. B. After [Goodenough et al, 1996].
All connexins have the common structure shown in Fig. 2. They are composed of
amino cytoplasmic terminal (NH 2 -), four membrane-spanning regions (M1-M4), a
cytoplasmic loop (CL), two extracellular domains (E1-E2) and a carboxyl cytoplasmic
terminal (COOH -). The N-terminal, M1-M4 and E1-E2 domains are well conserved
between different connexins. On the other hand, the C-terminal and CL domains are quite
divergent.
E1
M1
N
E2
extracellular
M3
M4
M2
CL
membrane
intracellular
C
Fig. 2: Connexin structure.
C. Connexins
As we told before, a connexin, is the constitutive element of a connexon. Different
connexins are observed in different tissues and confer particular characteristics to them.
Table 2 summarizes the most important connexins and the tissues or cells in which
they were observed. The table also shows the relationship between different connexins in
the form of a dendrogram that represents the primary sequence identity5.
5
A percentage related to the number of equal aminoacids in the protein chain.
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Cx33
testes
62%
Cx43
45%
Cx37
blastocyst, skin, lens, cornea
cardiac and smooth muscle,astrocytes
granulosa cells,fibroblasts, osteocytes
endothelium, corticalneuroblasts
39%
Cx40
endothelium,Purkinje fibers
46%
Cx50
lens fibers, corneal epithelium
42%
Cx46
lens fibers,Schwann cells
33%
Cx30.3
skin, kidney
56%
Cx31.1
stratified squamous epithelia
49%
Cx31
keratinocytes, kidney
49%
Cx26
hepatocytes, pancreaticacinar cells
keratinocytes,pinealocytes
62%
Cx32
hepatocytes,myelinatingSchwann cells
renal cells, neurons
29%
Cx45
lung, embryonic brain, kidney, skin,
heart
Table 2: Relationship between connexin family members and their location in tissues and cells. After
[Goodenough et al, 1996]
In order to name different connexins a systematic rule is applied. The connexins
are named by the species in which they occur and the predicted formula weight to the
nearest kilodalton [Bennett et al, 1991].
D. Physiology of Gap Junctions
Gap junctions play an important role in cell communication and signaling. We can
classify their functions depending on the excitability of the cells involved in the junction.
Excitable Cells
• Provide a faster mechanism for propagating action potentials than the
chemical synapses. Chemical synapses have a typical delay of 0.5 ms while
electrical synapses are considered to be instantaneous.
• Facilitates synchronous contraction of cells in muscular tissue by making up
a functional syncytium.
Non Excitable Cells
• The intercellular connection provides a means of buffering system that
smoothes fluctuations in ions and metabolites concentration in the ICL.
• Facilitates coordinated movements, e.g., cilia movement in epithelial tissue.
• In embryonic development provides positional information depending on the
concentration of certain substances.
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E. Gating of Gap Junctions
Once that we have summarize the functions of gap junction in intercellular
communication we briefly review the gating mechanism that leads to the opening and
closing states.
In Fig. 3 and Fig. 4 two models of the gating mechanism of gap junctions are
presented. The basic idea is that the connexins that make up the connexon can undergo
conformational changes leading to the opening or closing of the gap junction channel. A
conformational change is a change in the stereochemistry of a molecule. In the model of
Fig. 3 it is shown that the difference between open and closed states is a rotation of the
connexins that reduces the effective channel width. In Fig. 4 a more complete view of the
mechanism is presented. From this figure it can be seen that the M3 domains of each
connexin line the interior part of the channel. By tilting of the connexin the channel
becomes narrower and the channel closes.
Fig. 3: Simple gating model. After [Alberts et al, 1993].
Fig. 4: More complex model indicating transmembrane
domains. After [Bennet et al, 1991].
F. Agents that Regulate Permeation of Gap Junctions
The permeability of gap junctions does not remain constant but, like voltage-gated
ion channels, it can be modulated according to certain variables. On the other hand, this
process is, in general, reversible.
As a general rule, it can be stated that the molecular weight of substances being
allowed to cross the junction should be less than 1kD as suggested by certain experiments
with dye molecules [Alberts et al, 1993].
The permeability of gap junctions is widely non selective allowing the pass of small
molecules (charged or not), second messengers and small metabolites. On the other hand
gap junctions are impermeable to nucleic acids and proteins [Bennett et al, 1991]. In
summary, their behavior is quite less selective that non junctional channels of the plasma
membrane (ionic channels) which makes that the coupled cells form a functional
syncytium.
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The main agents that regulate the permeability of gap junctions are the following:
• Voltage. Both transjunctional (Vj) and transmembrane (Vi-o) potential influence the
channel conductance. Gap junctions are not linear (they do not follow Ohm’s law).
If a channel (molecule) can exist in two states, say closed and open, and the energy
difference between these states is a linear function of voltage, then the equilibrium
distribution between these two states will be given by a Boltzmann relation of the
form:
pc / po = exp( A (V − Vo ))
where pc and po are closed and open probabilities, V is the applied voltage, Vo is
the voltage at which pc = po, A= nq (kT )-1 is a constant expressing gating charge,
and n is the equivalent number of unitary positive charges, q, moving through the
entire applied voltage. A more useful form of the relation is
po = 1 / (1 + exp( A(V − Vo))
If the closed state conductance is zero, gj=Nγpo where N is the number of channels.
Bennett and Verselis [Bennett & Verselis, 1992] also comment that in most cases
of voltage dependence gj does not go to zero but, instead approaches a minimum
value. By accepting this latter idea, they propose the following expression:
g j = ( g j max − g j min ) /( 1 + exp( A(V − Vo))) + g j min
Up to this point is has been supposed that conductance has a symmetrical
voltage dependence. However, in heterotypic junctions6 a rectifying behavior is
observed.
Fig. 5: Voltage sensitivity of gap junctions between pairs of amphibian blastomeres. After [Bennet & Verselis, 1992].
In Fig. 5, taken from [Bennet & Verselis, 1992], we show the voltage sensitivity
of special type of cell (blastomeres) in amphibian. A Boltzman distribution was
fitted for each voltage polarity . For this case Vo= 15mV and the effective charge
was about 6 which is comparable to that of channels of electrically excitable
membranes.
• Lipophiles. Several lipophilic7 compounds are among the pharmacological agents
that block electrical communication. The main compounds investigated were8:
6
Junctions with two connexons made up of different connexins.
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• Octanol and heptanol: these substances reversibly block conductance in a
dosedependent manner9.
• Volatile Anesthetics (halothane and ethrane): applied in mM
concentrations reversibly block cardiac gap junctions.
• Arachnidonic acid, unsaturated fatty acids and doxyl stearic acids: block
junctional conductance at relatively low (µM) concentrations.
All of these lipophilic agents are likely to act by incorporation into the lipid
bilayer.
• Cytoplasmic pH and Ca2+ concentration. Cytoplasmic acidification or increase
of [Ca2+ ] lead to a reduction in junctional conductance in many systems. Although
the influence of pH in junctional conductance is important under physiological
conditions, the influence of variations in channel conductance due to Ca2+ is only
important under pathological circumstances.
An interesting interpretation for Ca2+ dependence is as mechanism of cell
defense. Suppose that a diseased cell begins leaking out its cytoplasmic material
because a rupture in its plasma membrane. As a consequence of this, Ca2+ begins
entering the cell because it is more concentrated in the ECL and valuable
metabolites of the ICL are lost. Gap junctions close avoiding in this way that this
unbalance in the chemistry of the intracellular space may affect the healthy
neighboring cells.
• Phosphorylation. [Goodenough et al, 1996; Bennett & Verselis, 1992] The action
of kinases to phosphorylate connexins and that of phosphatases to
desphosphorylate them are mechanisms that seemly influence the energy barrier
between open and closed states of gap junctions. In this way, they can modify the
effective conductance of channels. The main problem is that different kinase affect
junctional conductance differently in different tissues expressing the same or
different connexins.
Table 3 summarizes different kinase action following the work of Bennett et al
[Bennett et al, 1991]:
7
Substances which can be dissolved in non polar solvents.
8
Cf. [Beyer, 1993] for original references on each substance group.
9
Takens-Kwak [Takens-Kwak et al, 1992] found that heptanol reduced all non-junctional
membrane ionic currents, in addition to gap junction conductance. These authors propose that
heptanol affects the membrane lipid structure rather than interacting directly with gap junctional
channels.
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Kinase
Cell Type
Internal Report 97-01
Junctional
Conductance
↑
↑
↓
↓
↓
Connexin
Reference
32 and 26
43
43
43
?
43
43
32 and 26
?, 43+?
43
?
Sáez et al., 1986, 1990
De Mello, 1988, Burt & Spray, 1988
Cole & Garfield, 1986
Grassi et al., 1986
Piccoline et al., 1984, DeVries &
Schwartz, 1989, McMahon et al., 1989
Grassi et al., 1986
Spray & Burt, 1990
Sáez et al., 1990
Trosko et al., 1988, Chanson et al., 1988
Crow et al., 1990
Musil et al., 1990
Musil et al., 1990
cAMP-dependent
protein kinase
(A kinase)
Hepatocytes
Cardiac Myocytes
Myometrium
Sertoli cells
Horizontal cells
Protein kinase C
Sertoli cells
Cardiac myocytes
Hepatocytes
Cell lines
cGMP-dependent
protein kinase
(G kinase)
Cardiac myocytes
Horizontal cells
↑
↑
?
↓
↓
↓
Tyrosine kinase
Fibroblast
↓
Unknown
Cardiocytes,
leptomeningeal cells
Lens
Cell lines
?
43
43
?
↑
43
43
Burt & Spray, 1988
DeVries and Schwwartz, 1989
Hertzberg et al., 1989
Table 3: Kinase action on juntional conductance.
G. Conductance of Gap Junctions
Macroscopic Junctional Conductance (voltage clamp): a method for assessing
macroscopic junctional conductance is by voltage clamping two interconnected cells as
shown in Fig. 6 where an electric model for two cells is presented. In this figure, gj, g nj1
and gnj2 stand for the junctional and non junctional conductances involved.
Cell 2
Cell 1
∆ V1
∆I 2
gj
∆I 1
gnj1
gnj2
∆ V2=0
Fig. 6: Simplified model of voltage clamp method.
By applying a voltage step ∆V1 to one of the cells (to cell no 1 in the figure) and a
current ∆I2 to the other cell (cell no 2) that equals the junctional current, it is possible to
maintain the last cell to a constant potential. In this way, it is possible to obtain the value
of both junctional and non junctional currents as:
g j = − ∆ I2 / ∆V1
gnj 1 = (∆ I1 + ∆ I2 ) / ∆V1
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In a similar way can be estimated the non junctional conductance of the second cell.
Single Channel Conductance of Gap Junctions (patch clamp): recording of
single gap junction channels is possible by using cells with non-junctional membranes of
very low conductance. By patch clamp techniques [Sakmann & Neher, 1984] it was
possible to show that many cells have non junctional conductances in de nanosiemens
range and single gap junctions have unitary conductance (γ) readily detectable against that
background [Bennet & Verselis, 1992]. A simple procedure for gap junctions is to hold
two coupled cells at different potentials; under these conditions opening of a junctional
channel is signaled by an increase in current that is equal and opposite in the two cells. A
single channel event in non junctional membrane is signaled by a current step in that cell
alone. In Table 4, adapted from [Bennet & Verselis, 1992], some typical values for single
channel conductance are shown.
Unitary conductance (pS)
Tissue:
Rat lacrimal gland
Rat neonatal heart
Chicken embryonic heart
Guinea-pig heart
Hamster ovarian cells
Pancreatic acinar cells
Earthworm MGA septum
Astrocytes
Horizontal cells
Xenopus embryonic muscle cells
Rat leptomeningeal cells
WB cells
Exogenous expression
SKHep1 transfected cells
Cx32
Cx43
Artificial bilayers
Isolated rat liver junctional membranes (Cx32, Cx26)
Isolated junctional membranes
Solubilized Cx32
110-130
50, 45
60-80, 165
30-40
22-120
27, 130
100
50-60
50
100
40-90
80-90
Connexin type
Cx32?, Cx26?
Cx43
Cx42, Cx43, Cx45
Cx43?
Cx43?
Cx32
?
Cx43
?
Xen 43
Cx43, 34kD protein
130,150
60, 90
150
130-160
50, 130
Table 4: Single-channel conductance of different tissues. Adapted from [Bennett & Verselis, 1992]
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III. GAP JUNCTIONS AND F AILING HEART
A. Cardiac Muscle Tissue
Animal body is organized at different levels. At tissue level, four different kind of
tissues can be distinguished: epithelial, nervous, connective (proper, vascular or blood,
osseous or bone and cartilage) and muscle. There are three types of muscular tissue:
skeletal muscle, smooth muscle and cardiac muscle. In fact cardiac muscle tissue has
some characteristics in common with both skeletal (both are striated muscles) and
smooth muscles (both are involuntary ).
Cardiac Muscle Tissue:Description: branched cylindrical, striated fibers with one or two centrally
located nuclei; contains intercalated discs; mainly involuntary control.
Location: heart wall.
Function: pumps blood to all parts of the body.
Fig. 7: Cardiac muscle tissue. From [Tortora & Grabowsky, 1993].
We will briefly review the organization of cardiac muscle tissue in order to correctly
locate gap junctions. Cardiac muscle is composed of an arrangements of cells called
myocytes. They are branched cylindrical cells with striated fibers responsible of the
contractile properties of the tissue. Each myocyte may contain one or two centrally
located nucleus. Mitochondria may be quite large in cardiac muscle. They are typically 2.5
µm long but may occasionally arrive to 7 or 8 µm. Another important aspect is the
existence of the transverse tubular system which is composed of invaginations of the
sarcoplamic membrane. This system allows easy propagation of action potential through
the interior of the myocyte in order to facilitate the synchronous contraction. It is said that
every interior point is within a range of 2 to 3 µm of the extracellular space [Fawcett,
1986]. Fig. 7 shows a simplified diagram of the structure of a myocytes’ network.. Fig 8
shows a diagram of the interior of a myocyte based on an electron micrograph.
Cardiac muscle fibers branch and interconnect with each other, but form two
separate networks. The muscular walls and partition of the upper chambers of the heart
(atria) compose one network. The muscular walls and partition of the lower chambers
(ventricles) compose the other network. Each fiber in a network is connected to its
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neighbors by irregular, transverse thickenings of the sarcolemma10 called intercalated
discs (Fig. 9).
Fig. 8: Diagram of the interior of a myocyte based on an electron micrograph. After [Tortora & Grabowsky, 1993]
Fig. 9: Detail of the junctional portion of myocytes.
The intercalated discs contain three types of junctions: desmosome and fascia
adherens11 which are responsible of myocytes’ adhesion -anchoring junctions- and gap
junctions which allow easy spread of action potentials through the network that works as
10
Sarcolemma and sarcoplasma are the names of the cytoplasmic region and plasma
membrane of muscle cells.
11
Fascia adherens (sometimes also called zonula adherens) is a special type of adhesion belt
junction.
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a functional unit (syncytium). The main connexin type found in normal human
myocardium is Cx43. Each myocyte interconnects with about 11 other myocytes through
its branches [Severs, 1994a]. Desmosomes and fascia adherens are mainly located in the
transverse regions of the intercalated discs while gap junctions’ macula are more
concentrated in the longitudinal faces between myocytes. Both desmosomes and fascia
adherens have a much wider intermembrane space (20-25 nm) than gap junctions (2-3
nm) [Severs, 1994a] . Myocytes are roughly quadrangular when viewed in a transverse
cut, and about 14µm diameter [Tortora & Grabowsky, 1993].
The quantity and distribution of the gaps in the heart follows the functional needs.
While in the atrioventricular node there are few and sparse gaps in order to increase the
delay in the action potential propagation, Purkinje fibers and active myocardium are reach
in gaps.
A final observation about action potential propagation is its anisotropy. In normal
myocardium conduction parallel to the longitudinal axis of the myocytes is three times
faster than in the transverse direction [Severs, 1994a]. This may be because gap junctions
are predominantly distributed in the longitudinal pathway joining myocytes. Moreover the
ratio between the cytoplasmic membrane resistance and the resistance due to gap
junctions may be as large as 400 [Guyton & Hall, 1996].
B. Ischemia and Myocardial Infarct
In next section we will summarize some experimental results about gap junctions
in normal and failing heart. By failing heart we will consider ischemic heart. Ischemia is
a heart fault arising from a reduction of blood flow in a certain region of the heart.
Ischemia usually causes hypoxia which is a reduction in oxygen supply that may weaken
cells without killing them. The main consequence of a poor blood supply is a reduction in
glucose and oxygen concentrations in the extracellular fluid.
More serious than ischemia is myocardial infarction, commonly called heart
attack. Infarction means the death of an area of tissue because an interrupted blood
supply. Myocardial infarction may result from a thrombus (stationary blood clot) or
embolus (blood clot transported by blood) in one of the coronary arteries. The tissue distal
to the obstruction dies and is replaced by non contractile scar tissue. Thus the heart
muscle loses at least some of its strength being the aftereffects dependent upon the size
and location of the infarcted or dead area.
C. Gap Junction Alterations in Failing Heart
Severs [Severs, 1994a; 1994b] reported two main alterations in gap distribution and
quantity in ischemic myocardial tissue:
• Disturbance in the spatial distribution of gap junctions at the border zone of healed
infarcts.
• Reduction in the quantity of immunodetectable Cx43 in regions of normal gap
junction distribution distant from infarct scar.
Gap junctions in ischemic tissue are not confined to the intercalated discs but may
be sparsely distributed in the cellular surface. The myocytes presenting this alteration may
be several hundred micrometers from the border of the infarct scar. They may be even
mixed among apparently normal myocytes.
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There is also a reduction in the number of gaps per unit length and in their size
selectively affecting long gap junctions detected in transverse planes of section. The
number of intercalated disc contacts per myocyte is markedly reduced, a change largely
confined to the discs making side-to-side connections between cells (those in branches
rather than at the ends of the main body of the cell).
All this effects may contribute to the anisotropic propagation of the wavefront by
reducing the coupling between myocytes and stressing the difference in propagation
speed of the action potential in the longitudinal and transverse directions.
Although myocardium distant from infarct scars has seemingly normal
arrangement of gap junctions, the occurrence of ventricular arrhythmias in other clinical
settings, such as hypertrophy, raises the question as to whether the diseased heart is
afflicted with a more wide-spread gap-junctional abnormality.
It seems that reduced expression of connexin 43 may be a key factor. It has been
shown that this reduction in expression is not only confined to ischemic heart disease.
This abnormality may be also found in non ischemic hypertrophied myocardium.
It has also been reported that cultured myocytes infected with Trypanosoma cruzi,
the unicellular parasite responsible for Chagas’ disease (the most common cause of heart
disease in South America), show reductions in gap junctional conductance and
immunocytochemically detectable connexin 43 levels. The occurrence of a common gapjunctional abnormality in the form of reduced connexin 43 in these diverse disease
settings, all of which are associated with an arrhythmic tendency, is highly suggestive of a
direct link between connexin 43 levels and conduction disturbances.
IV. ACKNOWLEDGMENT
This report was carried out under a CIRIT grant. I thank Dr. Ruth Ferrer (School
of Pharmaceutics, Universitat de Barcelona) for her useful comments.
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V. REFERENCES
Alberts B, Bray D, Lewis J, Raff M, Roberts K, Watson J, 1994, Molecular
Biology of the Cell, 3rd Ed., Garland Publishing Inc.
Bennett MVL, Barrio LC, Bargiello TA,Spray DC, Hertzberg E, Sáez JC, 1991,
“Gap Junctions: New Tools, New Answers, New Questions”, Neuron,
6:305-320.
Bennet MVL, Verselis VK, 1992, “Biophysics of Gap Junctions”, Sem. Cell
Biol., 3:29-47.
Beyer EC, 1993, “Gap Junctions”, Int. Rev. Cytol., 137C:1-38.
Fawcett D, 1986, A textbook of histology, 11th Ed, WB Saunders Co.
Goodenough DA, Goliger JA, Paul DL, 1996, “Connexins, Connexons and
Intercellular Communication”, Ann. Rev. Biochemistry, 65:475-502.
Guyton AC, Hall JE, 1996, Textbook of Medical Physiology, 9th Ed, WB
Saunders Co.
Matthews GG, 1994, Cellular Physiology of Nerve and Muscle, 2nd Ed.,
Blackwell Scientific.
Sakmann B, Neher E, 1984, “Patch Clamp Techniques for Studying Ionic
Channels in Excitable Membranes”, Ann. Rev. Physiol., 46:455-72.
Severs N, 1994a, “Pathophysiology of Gap Junctions in Heart Disease”, J. Card.
Electrophysiol, 5:5(462-475).
Severs N, 1994b, “Gap Junctions Alterations in Heart Disease”, European Heart
J., 15:D53-57.
Takens-Kwak BR, Jongsma HJ, Rook MB, Van Ginneken AC, 1992, Am. J.
Physiol. 262:C1531-38.
Tortora GJ, Grabowsky SR, 1993, Principles of Anatomy and Physiology, 7th
Ed., Harpers Collins College Publishers.
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